In recent years, there has been a growing demand for larger display panels and smaller portable display panels. To this end, the power consumption of display elements must be reduced. In addition, transparent electrodes having high visible light transmittance and low resistance must be used.
Thus, an indium-tin-oxide film (hereinafter referred to as ITO film) in which an indium oxide is doped with several percent of tin is formed on a transparent substrate such as a glass plate to further decrease the resistance of a transparent electroconductive thin film (see, for example, Patent Document 1).
While the ITO film is highly transparent and has high electrical conductivity, the crustal abundance of indium is as small as 50 ppb. The cost of indium may therefore increase with the depletion of resources.
Furthermore, particularly in recent years, zinc oxide materials have been proposed as inexpensive materials having high plasma resistance.
However, zinc oxide materials are sensitive to acid or alkaline and erode gradually even in a carbon dioxide atmosphere. Although zinc oxide materials may be coated to improve their chemical resistance, the additional coating process causes an increase in manufacturing costs.
Hence, the transparent conductors must be composed of stably supplied materials having chemical resistance and high durability to extend the application range of the transparent conductors.
Under these circumstances, titanium dioxide (TiO2), which has both chemical resistance and high durability, has received attention (see, for example, Non-patent Document 1). For example, a TiO2 film is epitaxially grown on a sapphire substrate (see, for example, Non-patent Documents 2, 3, and 4).
However, when a sapphire substrate is used, a function of combining a substrate material and a different thin-film material (heterojunction) cannot be utilized. Thus, the sapphire substrate can negligibly function beyond being a simple structural material.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-95240
Non-patent Document 1: OYO BUTURI, 73, 5, 587-592 (2004)
Non-patent Document 2: Jpn. J. Appl. Phys. Vol. 40, pp. L 1204-L 1206 (2001)
Non-patent Document 3: Nature Materials 3, 221-224 (2004)
Non-patent Document 4: APPLIED PHYSICS LETTERS, VOLUME 78, NUMBER 18, 2664-2666 (2001)